The present invention relates to a magnetic recording medium, particularly a magnetic recording tape, comprising a base film, a magnetic layer provided on one surface (i.e. the major or front surface) of the base film and a back coat layer provided on the other surface (i.e. the back surface) of the base film.
In general, a magnetic recording tape comprises a base film and a magnetic layer provided on the major surface of the base film. In order to prevent various problems such as deterioriation of the running property and adhesion of dust caused by the electrification of the back surface, a back coat layer of low electrification is usually provided. While such back coat layer is effective in suppressing the electrification, its friction coefficient is higher than that of the back surface itself so that the running property is not significantly improved. Because of this reason, a lubricant is incorporated into the back coat layer so as to lower the friction coefficient.
As the lubricant for the above purpose, there is used a liquid or semi-solid lubricant, such as a fatty acid (e.g. stearic acid, myristic acid) or a fatty acid ester. There is also used a solid lubricant such as graphite powder or molybdenum disulfide powder. However, the use of the liquid or semi-solid lubricant in a large amount often makes the surface of the back coat layer tacky and stains the guide pin of a magnetic recording and reproducing device. Thus, a certain upper limit is present as to the amount of the liquid or semi-solid lubricant to be used, and therefore a satisfactory reduction of the friction coefficient is hardly attained. The use of the solid lubricant does not produce the drawbacks as seen in the use of the liquid or semi-solid lubricant. However, the solid lubricant is required to use in a relatively large amount for attaining a sufficient reduction of the friction coefficient, and therefore the surface smoothness is lost and the electromagnetic characteristics are deteriorated. This is due to the fact that the solid lubricant is usually manufactured through the step of crushing so that its fine particles are hardly obtainable.
In Japanese Patent Publication (unexamined) No. 88307/77, it is described that a graphite-like crystalline sturcture is partly observed in carbon black powder having a specific surface area of more than 700 m.sup.2 /g (determined by the N.sub.2 adsorption method (hereinafter referred to as "BET method")) and a DBP (dibutyl phthalate) oil absorption of more than 300 ml/100 g. Although an attempt was made to use such carbon black powder as the lubricant, its incorporation into the back coat layer did not exert such a desirable lubricating effect as initially expected; i.e. the carbon black powder did not show a sufficient lubricating effect without deterioration of the smoothness.